1. Field of the Invention
The present invention relates to organic nonlinear optical material, in particular, to a novel second-order nonlinear optical polymer and a method for producing the same.
2. Relevant Art
Recently, a technique for producing an optical modulator, frequency transducer, electric-field sensor probe or the like using a poled polymer has been investigated. The term "poled polymer" employed herein means a polymer obtained by poling a transparent polymer, which includes dye materials possessing enhanced second-order nonlinear optical properties.
In regard to the chemical structure of the poled polymer, it is preferable that dye molecules possessing a large second-order molecular hyperpolarizability be chemically bonded to a polymer backbone with a high concentration. The dye materials are generally .pi.-conjugated dye molecules with donors and acceptors.
The second-order nonlinear optical properties of some dye-attached polymers such as (a) an azo-dye-attached poly(methyl methacrylate) and (b) a stilbene-dye-attached poly(methyl methacrylate) have been reported. (See (a): K. D. Singer et al., Appl. Phys. Lett., 53, 1800 (1988) and (b): G. R. Mohlmann et al., Synthetic Metals, 37, 207 (1989)). It has been known that the second-order nonlinear optical constant is increased by increasing the amount of the dye molecules in the polymer. In addition, as a result of an attempt to increase the second-order molecular hyperpolarizability, a dye molecule including a long .pi.-conjugated chain with a donor and an acceptor, and possessing a large second-order molecular hyperpolarizability has been reported (see J. L. Oudar et al., J. Chem. Phys., 67, 446 (1977)). More concretely, a dye, having a conjugated structure formed of three aromatic rings linked by two azo groups positioned between the aromatic rings, can possess a larger second-order molecular hyperpolarizability, when compared with the stilbene dye or azo dye having two aromatic rings (see M. Amano et al., J. Appl. Phys., 68, 6024 (1990).
However, in regard to the poled polymer, in which dye molecules with long .pi.-conjugated chains (i.e. with many conjugated aromatic rings) possessing a large second-order molecular hyperpolarizability are bonded to the polymer backbone with a high concentration, there exist disadvantages such as inferior light transparency and poor optical uniformity in film formation. Such disadvantages are due to the fact that a polymer solution for film formation for use in a dip coating or spin coating process is liable to gelation, and for this reason, it is difficult to form a film possessing optical uniformity.